| Summary: | The effects of reproduction and maternal investment (i.e. milk transfer) on trace element levels remain poorly understood in marine mammals. We examined the blood dynamics of Cd, Pb, Hg, Cr, Ni, V, Zn, Cu, Fe, Ca and Se throughout lactation in a top predator from the North Sea, the grey seal (Halichoerus grypus). Trace element levels were measured in blood and milk of 21 mother-pup pairs in early and late lactation. Maternal hair and natal fur (lanugo) were also collected and analyzed. Trace element levels in blood decreased according to the following pattern: Fe > Ca > Zn > Se > Cu > Hg > Pb > Cr > V in mothers and pups. Cd and Ni were not detected in blood. The substantial levels measured in lanugo indicate nevertheless that all trace elements (including Cd and Ni) were transferred to offspring through the placenta. In maternal hair, Pb levels (~ 2.2 mg/kg dw) were relatively high in comparison with other phocid species. All trace elements, except Cd, were detected in the milk showing a transmammary transfer to offspring, especially for elements such as Ca, Se, V, Ni and Pb. Hg showed the highest levels in blood (up to 0.25 mg/kg ww) and hair (up to 21 mg/kg dw) from the non-essential and toxic metals measured. Levels of Hg and essential elements (Ca, Fe, Zn, Cu and Se) in blood and milk showed significant variations throughout the lactation period. Therefore, physiological processes such as lactation affect trace element levels in tissues of mother and pup elephant seals. Such processes and its relationship to body condition should be considered carefully when interpreting trace element levels in the framework of biomonitoring.
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